In air filters which are used for internal combustion engines, it is important to have substantially uniform air flow. This is because non-uniform air flow adversely affects the mass transfer function of the air stream, which decreases engine performance and raises exhaust emissions. In air filters, such as annular air filters, and more particularly, frustoconical air filters which are annular in cross-section, it has been found that reducing the height of a urethane seal adjacent the downstream clean side of the air filter improves air flow uniformity substantially. However, the prior art does not suggest just how this can be accomplished in an economical, feasible manufacturing process.
It is to be kept in mind that even a slight increase in engine performance and reduction in exhaust emissions is of considerable importance when that slight improvement is spread over an entire fleet of automotive vehicles. This is because that slight improvement, when combined with other slight improvements, eventually provides a substantial overall improvement which decreases fuel consumption of the fleet as well as harmful exhaust emissions of the fleet.
Generally, seals and gaskets formed on conical air filters are made of expanded urethane. It has been found that when attempting to lower the blow height of the urethane on the inside diameter of the filter media, sufficient flow or pressure from the foaming action is needed on the outside diameter of the filter media to form a blemish-free seal against a radial portion of a top mold. Moreover, since the outside diameter of the urethane flow is mechanically restricted and controlled, while the inside diameter is unrestricted, blow height is adversely impacted by the water content of polyol and the ratio of polyol to isocyanate, as well as other variables.
In an attempt to solve such difficulties, a number of methods were investigated which included manually restricting the blow on the inside, processing the urethane at different polyol to isocyanate ratios and different mold temperatures, and changing the chemistry of the urethane in order to reduce the blow. These methods failed due to either blemishes which formed on the outside diameter which forms the gasket for the filter element and/or high blow heights at the inside diameter of the filter element.